Are cams & compression independent variables?

[thechadzone]

Seems like I've read that they are dependent on each other for measurable results, but in an email I got from a very nice guy from Midwest Bayless, he wrote: "For what its worth, cams are not compression dependent. Duration of cam and compression are two independent variables which
both help greatly singularly or in tandem."

That seems to say that I can use my lumpy cams with adjustable pulleys in cooperation with my IDFs and stock 1608 pistons to create a desirable performance increase. ?

I'm sure people at Bayless know what they're talking about, but it seems like I've read contrary things here and other places. Anyone have any thoughts here? Can I summon Steve C?

[BEEK]

The larger the camshaft, duration wise, you are loosing dynamic compression as the valves are staying open longer .there is a trade off with overlap. The scavenging effect of the exhaust will also have some to do. This is a very basic explanation. So with "bigger cams" you do need to increase compression statically, because you will be loosing dynamic compression due to the camshaft. That’s why manufacturers recommend compression increases with bigger camshafts. You can get away with increasingly larger static compression ratio, with lesser octane fuel, due to the dynamic loss, "shouldn’t ping"
Take a 10-1 compression motor with stock cams; you will get a compression test of about 200lbs/sq in. change only the camshafts and the compression will decrease. I have had race motors that made only 175 lbs compression with 12-1 pistons, statically, but the dynamic was obviously way higher, as the motor made 200+hp at the flywheel. Again I have oversimplified the process and variables for a basic explanation

[Daniel]

What Cam Shafts were you thinking of using, the venders only offer smaller duration Cams .

[BEEK]

48-67 67-48 99.5°lobe centers 295°operating duration 96°overlap 11.35 lift. this is what i run in my performance motors, i advance the intake 1-2°, and retard the exhaust 2-3°, depending on dyno results, i also run a bit tighter lash than recomended, which increases the duration to over 300°.... just a fyi

[vandor]

I agree with Beek's explanation, and would add that hotter cams always take away from the bottom and add to the top. Increasing CR counteracts this to some degree.
I had a 124 Coupe with a stock CR 1800, IDFs and 35/75 cams, it had a lot of power over 5000 rpm, but not much low end torque.

[majicwrench]

I'll disagree....I think...
wilder cams will certainly lower the cranking compression due to overlap etc. BUT the trade-off is that at higher RPM ( if you could measure compression at 7000RPM) the compression would be HIGHER than with a stock cam. The very intent of a wild cam is to let in more fuel/air than you can get in with a stock cam. Which leads to higher pressures, which leads to more zoom.

You don't need to increase compression with hot cams. YOu can make loads of HP without high compression. Of course, high compression can help too. And you don't have to have wild cams to have high compression.
Like the guy said "Both help greatly, singularily or in tandem"

Cranking compression.... at what, 400rpm?? really means nothing, other than making sure all the cylinders are the same.
Keith

[BEEK]

Compression Ratio Explained (Static & Dynamic)


INTRODUCTION

If you look in the dictionary:

Compression as an adjective means something is squeezed (in this case it's air and fuel)

Ratio as a noun means something is divided by something else. It is a math term meaning a quotient.


There are 2 kinds of Compression Ratios (CR):

1. Static

2. Dynamic


The most common one everyone talks about is Static CR.

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1. STATIC CR



1A. DEFINITION

The Static Compression RATIO is defined as the Volume of the Combustion Chamber when the piston is at the very bottom of it's travel (called "bottom dead center" or BDC) DIVIDED BY the Volume of the Combustion Chamber when the piston is at the very top of it's travel (called "top dead center" or TDC).

Static compression ratio is one factor that affects how completely the air-fuel mixture is burned, once it has been lit by the sparkplug. If you burn all of the air:fuel mixture, you make more hp. If there is some leftover unburnt air:fuel mix after the spark has been lit, you have not gotten all the power you can get out of the mix that was just added into the cylinder. This completeness of burn is called THERMODYNAMIC EFFICIENCY (measured in units of energy called joules, pronounced like "jewels").

Remember, basically, you increase horsepower by increasing 3 different types of efficiencies: thermodynamic (relates to burn), volumetric (relates air flow in and out), and mechanical (relates to weight and friction). Improving thermodynamic efficiency is one of the 3 major power-gaining methods available for engine builders.

The relationship between thermodynamic efficiency and static CR is not a simple, direct, linear, straight-forward 1:1 relationship. In plain english, if you increase the CR by 1.5 times, it does not mean the burn efficiency (or completeness of burn) will also increase by 1.5 times. It's a complex direct exponential and inverse exponential equation that relates static CR to burn efficiency.

Like anything exponential and inverse in math, there is an initial rapid increase in thermodynamic efficiency as static CR increases but at some point of CR, the efficiency levels off and plateaus. In other words, at some point, further increases in static CR no longer improves or increases the burn efficiency.


1 B. THE PROBLEMS WITH INCREASING STATIC CR

The problem is that as you increase CR, you increase cylinder pressure and temperature inside the combustion chamber. When air is squeezed hard inside a closed container like a cylinder, the pressure inside goes up the harder you squeeze. As pressure builds up, so does temperature. These 2 (high pressure and temp.) can cause the air -fuel mix to ignite on it's own without a spark from the plug....this is called detonation. So there is a CR level which will cause detonation. The value of that level varies with each engine and depends on combustion chamber's design which limit detonation risk (eg. compact combustion chambers, low surface-area-to-volume ratio combustion chambers, more quench or squish area, swirl filling of the air-fuel mix into the chamber, getting a stratified air fuel mix once the chamber is completely filled, cooling ability of the engine, etc. ... all of these other chamber design factors reduce detonation risk and speeds up the burn rate, against the effect of higher static CR ).

Secondly, as you increase static CR more and more, the cylinder pressures increase more and more. The piston must work much harder to compress the same amount of air:fuel mixture delivered into the chamber due to this higher pressure. This negative work adds more power-robbing or parasitic-losing friction and slows the piston speed momentum which affects the power you produce.

So you can make more power by improving burn efficiency via increasing the static CR up to a point. For street engines, the maximum static CR on pump gas is around 12.5:1 CR if you know how to tune. If you do not, the maximum is around 11.5:1 CR. For a race engine, the point at which cranking pressure causes negative work or parasitic friction and affects power output is around 14:1 CR. Alcohol-fueled race engines can afford to run 15-17:1 CR, since the alcohol cools the chamber and lowers both the cylinder temperature and detonation risk. Methanol race engines run much richer air-fuel ratios (around 5-6:1) than gasoline engines, as well.


In simple terms: for static CR, the first number is the chamber volume at BDC, second number is chamber volume at TDC...the higher the first number is, the more squeeze you have. More squeeze improves burn up to a certain point. This point varies for each engine design.


When we talk about turbos and superchargers, the point at which an increase in static CR that can cause detonation is much lower than in all motor engines.

The reason is : you are stuffing in more air into a closed container with boost. The cylinder pressures build up to a higher level faster. And so does temp. So the risk of detonation is much higher.


If you change cylinder heads (which have different head volumes) or change pistons (with different dome heights), or crankshafts (with different strokes), you will change both the static CR and the dynamic CR.





1C. HOW TO CALCULATE STATIC CR FROM SCRATCH


In not-so-simple terms, for the more math-inclined members, you can also calculate this out yourself (i.e. to understand the definition of CR more clearly and what individual factors determ it):




CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

CR = Compression Ratio

D = Displacement

PV = Piston Volume

DC = Deck Clearance Volume

G = Gasket Volume

CC = Combustion Chamber Volume




More Detail:

Static Compression Ratio = (Volume at BDC) / (Volume at TDC)

[ Aside: Volume stated in this equation above is the combustion chamber volume. ]


where


Volume at BDC = [ (Swept Volume + Cylinder Head Volume + Headgasket Volume (including piston to deck height) - Piston Dome Volume ]

Volume at TDC = [ Cylinder Head Volume + Headgasket Volume (including piston to deck height) - Piston Dome Volume ]

Swept Volume = [Pi x [Bore] ^2 x Stroke]/4000

Call the piston manufacturer for the aftermarket piston dome displacement spec.


2. Dynamic CR

The piston is always moving up and down but the intake valve opens and closes during this time as well.

As the piston is beginning to squeeze at BDC, the intake valve is beginning to close. The intake valve is not completely shut until the piston is near TDC. There is a connection between the cylinder combustion chamber and the intake port/intake manifold runner, when the intake valve is still partially open. As the piston is squeezing and approaching TDC, some cylinder pressure can bleed up in the intake port which reduces overall cylinder pressure.

If you use your adjustable intake cam gear to close the intake valve earlier (advancing the cam gear), the amount of cylinder pressure bleeding up the intake port is reduced. The cylinder pressure builds up faster and you get a better burn.

If you let the intake valve close later (retard the intake cam gear or use a longer duration intake cam), more cylinder pressure will bleed out or be reduced and the burn will be less complete.

This is why it is important to increase your static CR when you get extremely longer duration cams.

You want a fast, complete burn of the air fuel mix to make power.


Remember when we talk about CR, the first numb is the volume at BDC, second number is volume at TDC...the higher the first number, the more squeeze...

[narfire]

Thanks so much for that. Even a dumb old ex logger can make sence of your explination. I like the part about the intake cam closing before the piston comes up before detonation. I've put some cams (marks 274 FI ) in my car and love the improvement but I had the engine degreed in with the adjustable cam gears. I'll have to ask the fellow where the valves are at btc before ignition stroke. Perhaps there is even a bit more to be had. I'm going to take the car to a chassis dyno perhaps this friday to see what the car is doing.(weather permiting)
Love it
Chris

[thechadzone]

Thanks to all! Glad to ask a good question, even if there is no "easy" answer. I guess if it were easy, everyone'd have a bitchin' Italian twin cam...

[Daniel]

for street fiat engines 3 degrees advance seems to be the magic number if your not going past 7k on most cams
i've tuned .

[fiatfactory]

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